The present invention relates to a thermoplastic resin composition having high resistance to attack by chemicals such as halogenated hydrocarbons and especially hydrochlorofluorocarbons (HCFC's), good thermoforming and film formation characteristics and good recycling capabilities. More particularly, the present invention relates to such a composition and to thermoformed objects therefrom.
It is previously known in the art to prepare thermoformable sheets from thermoplastic resins such as impact modified polystyrene (HIPS) and impact modified styrene acrylonitrile (ABS) resins. Such thermoformable resins have found wide use in the preparation of numerous objects such as containers, toys, and appliance components. In particular, refrigerator or freezer liners are often thermoformed from such a sheet material. In the manufacture of a typical refrigerator or freezer, such a liner is placed into an outer jacket formed of metal or similar material that forms the outer cabinet or door of the refrigerator or freezer, and the space between the outer jacket and the liner is filled with a thermosetting foam, usually polyurethane foam. Typically, polyurethane foams are prepared by blending a polyurethane forming mixture comprising a polyhydroxyl compound and a polyisocyanate compound with a suitable blowing agent and allowing the viscous liquid mixture to expand due to the effects of the blowing agent while the reaction to form the polyurethane takes place. By injecting the polyurethane forming mixture in the free space between the refrigerator or freezer liner and the outer jacket, and allowing the mixture to foam in place, a unitary structure is formed that greatly aids in the rigidity of the resulting refrigerator or freezer. Varying degrees of adhesion between the polyurethane foam and the liner may be obtained during the foaming process.
A number of chemicals are suitably employed as blowing agents, especially halogenated hydrocarbons, many of which are aggressive solvents for vinyl aromatic polymers. Recently, hydrochlorofluorocarbon (HCFC) and hydrofluorocarbon (HFC) blowing agents have been employed in a number of applications due to the reduced environmental degradation believed to result from releasing such blowing agents to the environment. Some HCFC and HFC blowing agents (also referred to as soft blowing agents) have been found to be highly aggressive solvents with respect to many thermoplastic resins, particularly styrene-containing resins, typically employed in thermoforming compositions. Due to this increased solubility, greater chemical attack of the thermoplastic resin can result if HCFC blowing agents come in contact with the surface of such thermoformable resin during the manufacturing process or while in use. This can lead to blistering and/or cracking of the thermoplastic resin due to the chemical attack at the point of contact with the thermoformed sheet.
Attempts to overcome such aggressive attack by compounds on typical thermoformable sheet materials have included use of an intervening barrier layer which may be coextruded or heat laminated to the thermoformable sheet material. Other solutions, such as coatings, similarly have not proven acceptable. Special resins which have improved resistance to the effects of HCFC's have also been attempted, but often such resins possess poor thermoformability, inadequate strength, inability to include reground scrap material from the manufacturing process, high cost or other unacceptable property.
For example, U.S. Pat. No. 4,386,187 discloses a resin blend having improved chemical resistance, however, it has now been discovered that such resins as are disclosed by this reference are not continuous in the vinylaromatic polymer phase and demonstrate poor thermoformability and insufficient flexural modulus and other physical properties for applications as a monolayer structure, such as refrigerator or freezer liners.
In many thermoplastic manufacturing processes, the use (recycling) of regrind or in-plant scrap is a routine part of the operation. Resin processors usually mix the regrind with virgin resin or use it by itself. No matter which discipline is followed, the regrind product sees several heat histories before it ends up in the finished article. In addition, with the efforts to recycle post-consumer plastic waste, these same resins that contain regrind product may be recycled and exposed to additional processing.
In ternary blends containing styrenic block copolymers with butadiene midblocks, repeated processing often leads to crosslinking which results in increased viscosities, processing difficulties and, ultimately, reductions in physical properties of the blend. The crosslinking is affected by the temperature of processing and limits the temperature at which the blends may be processed. In blends containing isoprene midblocks, chain scission, or depolymerization, predominates and the polymer tends toward reduced viscosities. The most common approach to address these potential changes is restabilization of the regrind product. To this end, manufacturers and resin producers have enjoyed only limited success in developing formulations which can withstand the heat histories of multiple exposures to melt processes.
Accordingly, it would be desirable if there were provided a thermoformable, thermoplastic resin having improved resistance to attack by halogenated hydrocarbons, especially hydrochlorofluorocarbons, which is capable of being processed and reprocessed (recycled) without loss of appearance, properties and/or physical properties.
What is needed is a thermoplastic polymer composition which can be recycled several times without deleteriously affecting the properties of products made from such polymer compositions. What is further needed is such a thermoplastic polymer composition which can be recycled without the need for additional or increased stabilizer levels. What is also needed is such a polymer composition wherein the processing conditions are not unnecessarily limited due to degradation of one component of the blend.
In addition, it would be desirable if there were provided a sheet material comprising such a thermoformable thermoplastic resin.